Two-way audio communication



Feb. 2, 1965 s. H. HARRISON TWO-WAY AUDIO COMMUNICATION 2 Sheets-Sheet 1 Filed July 13, 1962 m y All m m 2 2 I o Y R 6 i m R M N W 0 wfl v fl A J A/ M V m mSmmNEQQw 5 Y vfiazm B i m H 205cm Feb. 2, 1965 s. H. HARRISON TWO-WAY AUDIO COMMUNICATION 2 Sheets-Sheet 2 Filed July 13, 1962 NEE TEE/x mowmmmmabm dmazqaxm -m mh Fae umiimzq mzi mommmmmmbw MEL 15:3 MmRQQ m Mu NEE 312i IN VEN TOR. (Sm/5w 1499/5 O/V M M A TTORNE Y United States Patent 3,168,619 TWO-WAY AUDIO COMMUNICATIDN Stevens H. Harrison, Brooklyn, N.Y., assignor to Bankers Trust Company, New York, N.Y., a corporation of New York Filed July 13, 1962, Ser. N 2tl9,48 18 Claims. (Ci. 1791) This invention relates to communication systems, especially systems providing two-Way communication of voice or the like, notably where it is desired to utilize loud speakers for reproduction, and microphones that need not be placed in immediate proximity to persons talking. In a specific sense, the invention is directed to the provision of novel and effective gain switching means and operations in systems of the described type whereby certain difficulties incidental to these systems, e.g., as employed with continuously open microphones, are avoided.

By way of example, it has long been desired to provide a voice intercommunication system, or so-called intercom, where persons at one station may converse with persons at another station, through the instrumentality of remotely situated microphones at the respective stations, and loud speaking reproducers. If feasible, such arrangements would permit conferences and discussions to be carried on between groups of people, Without the need for any person to speak immediately or directly into a microphone before him, and with everything said at either station being heard plainly by all persons at the other station. A special use of this type of communication as in conjunction with two-way closed circuit television conference circuits whereby the person or persons at each station, for example as sitting around a conference table, may continuously see the persons at the other station, the entire effect being that of a single conference or meeting. The Voice or audio communication system, however, to which the present improvements are directed, is nevertheless highly useful in itself, i.e., apart from any accompanying video system.

While advantages of this type of operation may have been appreciated, its accomplishment has been hindered because of the problem of acoustical electrical feedback, and resulting howl or the like as occasioned by the remote positioning of the microphones at the respective stations. That is to say, with the microphone sensitive to voice at remote parts of the room, incoming signals from the loud speaker tend to be picked up as well as desired speech from the persons present, with the result that there is audible feedback howl occurring in'the loop formed by the two channels, destroying the utility of the system. If the amplification level or gain in the channels is reduced to avoid feedback, the reproduction levels of sound generally become too low for practical use.

Manually operated level shifting can of course be employed to disable, or partially disable, one line when the other is in use for speech, but such operation is cumbersome and inconvenient and causes annoying interruptions of the oral discussion or conference between the stations. While it has been proposed to employ electro-mechanical relays in an arrangement that would be responsive to speech signals, for reducing or disabling the communication in a currently unused direction between the stations, such proposals are understood to have been found unsatisfactory in practice, e.g. as involving unpleasantly abrupt changes or breaks in the flow of conversation, or as otherwise exhibiting deficient characteristics of so-called attack and recovery time in operation. Moreover, no simple or effective electronic means to accomplish such result appears to have been available, it being further understood that any effort to solve the problem by electronic circuits proposed for other pur- 3,168,619 Patented Feb. 2, 1965 poses would involve relatively complex and expensive arrangements.

Accordingly, the present invention is directed to a new and highly effective communication system embodying what may be described as gain switching operations, whereby level or gain control is automatically and unobtrusively achieved so as to permit full realization of a fully open two-way system, e.g. having loud speaking r production and remote microphones, while entirely avoiding feedback howl. The advantages of such a system, eg. as used either with or without an accompanying two- Way video circuit, are essentially as explained hereinabove.

Corollary advantages, beyond the results of convenience and or completely natural conversation, are avoidance of unwanted noises. If microphones must be placed close-in, i.e. on a table immediately in front of the speaker, as in conventional arrangements, the noises of paper rattling, tapping, and other incidental sounds are unduly picked up and transmitted, creating an abnormally loud background in reproduction at the other station. In such arrangements, persons speaking are too often apt to move or turn away from the microphone, with resulting attenuation or loss of speech transmission. The arrangements of the present invention, as explained below, avoid all of these difiiculties, and permit the use of a microphone placed remotely in the room, so that the speech of everyone who talks is fully transmitted, and incidental noises are heard to no greater extent than by persons present. Indeed in the present systems full range audio equipment, with good volume, can be employed without any special acoustical treatment of the conference or other rooms that are used; nor is it necessary to employ special band pass networks or the like which are sometimes utilized in communication channels to reduce feedback, but which result in objectionable distortion of speech or other sound.

To these and other ends, the present invention embraces an organization of gain switching instrumentalities which may be conveniently described as expanders and suppressors and which are controlled or actuated, in a novel manner, through the instrumentality of so-called light-dependent resistors. In particular, the gain switching operation is based on the light-dependent characteris tics of the stated resistors, specifically cadmium sulfide cells, which are used in association with signal-driven lamps as level control elements applied directly in each channel of the communication system. Thus considering a given channel in which voice is to be transmitted, the present apparatus includes an expander having a lightdependent resistor for its control, and also includes provision for supplying a signal to a drive amplifier or the like, that in turn is arranged to energize a lamp or equivalent illuminating means associated with the stated resistor. At the same time the other channel includes a suppressor, conveniently herein described as a cross-suppressor, which also embraces a similar light-dependent resistor as its control element, arranged for exposure to illuminating means. The last-mentioned illuminating means or lamp is identically controlled by voice signals in the first channel, e.g. being the same or a similar lamp as is employed to govern the expander. Hence when a speech signal is received in the first channel, the lamp or lamps are triggered, simultaneously switching the gain in both channels, to increase or expand it in the first and to suppress, i.e. reduce it in the second.

Complete systems according to the invention include an expander of the character described, in each channel, and likewise a cross-suppressor, operated by the signal from the other channel, whereby the voice signal in either selected channel is effectively transmitted at a high level,

n 6 while the level in the alternate channel is reduced, with the overall efiect of completely avoiding any feedback because of the acoustical path between the open speaker and microphone at either station.

Stated in another way, the circuit provides automatic gain switching for eiiective communication in either direction at will, using high levels of amplification in the speaking channel, but without the feedback howl that would otherwise be encountered with continuously open transducers. As explained, the circuit ispredicated on the characteristics of light-dependent resistors, e.g. of the cadmium sulfide type, which are employed in association with signal-driven lamps as level control elements in the respective channels. Specifically, the gain switching, triggered by voice in the speaking channel, accomplishes expansion in that channel and simultaneous cross-suppression of the opposite channel, the neteilect of both operations being to keep the total gain of the two-channel intercom loop below the feedback threshold while permitting high level amplification in the working channel.

The light-dependent (cadmium sulfide) cells are essentially simple resistive elements that exhibit a very large decrease in resistance upon and during exposure to light. Such resistor affords an uncomplicated, economical control element which introduces no distortion nor frequency discrimination. In the presently preferred circuits, the resistor is included as the variable leg of a simple resistive voltage divider, arranged and included in the signal path in one or another of suitable ways for the desired func tions of effecting expansion or suppression, i.e. when triggered by a simple lamp of the discharge (or in some cases, incandescent) type energized by a drive amplifier that is fed by the voice signal in the channel where gain is to be switched to a higher level. The lamp unit can be a conventional high gain voltage amplifier.

The dynamic characteristics of the gain switching circuit are unusually appropriate. The so-called attack time, in which full expansion occurs, is of the order of ten milliseconds, which is fast enough to avoid any initial clipping of first syllables, e.g. after pauses in speech. The mutual recovery time in expansion and cross-suppression, i.e. after disappearance of a signal to the lamp-drive amplifier, requires up to several seconds, so that there is a gradual level shift between the two channels. This is an important characteristic because abrupt jumps in the ambient room noise heard through the system would be disturbing to the listeners.

The accompanying drawings illustrate certain embodiments of the invention, set forth by way of example as follows:

FIG. 1 is a wiring diagram, with conventional parts in schematic illustration, of one form of intercom embracing the gain switching means of the invention;

PEG. 2 is a block diagram, showing an embodiment of the invention in another form of communication system; and Y FIG. 3 is a further modification of the invention as applied to a system of the general sort shown in FIG. 2.

Referring to FIG. 1, it will be understood that the system is designed for intercommunication between station I and station II, i.e. utilizing a microphone 1t and speaker 11 at the former, and a microphone 12 and speaker 13 at the latter. The microphones may conveniently be of the so-called long-throw type which are especially suited for pick-up over considerable distances, of the order of feet or more, it being contemplated that persons situated at station I will converse with persons at station 11 while situated in conventional positions or localities at each station, eg as for conversation among themselves and without being required to speak closely into either microphone.

Channel A, connecting microphone it) with speaker 13, comprises conventional conductors or transmission cables and conventional amplifying means such as the microphone pre-amplifier 14 and the loud speaker power ampliher 15, each of these being electronic devices of well known type. Similarly channel B extending from microphone ]12 to speaker 11 may be identically constituted, for instance as including a pre-amplifier 16 and loud speaker amplifier 17. It will be understood that with these components alone, the level of amplification in the two circuits cannot be raised sufiiciently for satisfactory communication, i.e. where the people at the two stations are talking at localities remote from the microphones; unless the gain is reduced to a very low level in each channel, feedback occurs, producing the usual howl and destroying communication.

In accordance with the present invention, channel A includes an expander circuit 2% consisting of a voltage divider. The shunt leg of the divider is the resistance 21 connected across the input of the pro-amplifier 14. The series leg of the divider consists of a fixed resistance 22 connected in parallel with a light-dependent resistor 23, one end of this leg being connected to one side of the line from the microphone it) and the other end of the leg being connected to one side of the shunt leg resistor 21, i.e. so that the two legs of the divider are connected in series across the output of the microphone it It may be explained that in this and other parts of FIG. 1 the conventional conductor elements of the line in channel A (and likewise in channel B) are drawn as simple, single lines, omitting ground returns, which would in fact be appropriately connected, for example, to the central terminal 24 of the expander network, i.e. the ground terminal of the voltage divider.

The output of the microphone it thus appears across the entire voltage divider, while the input of the pre-amplifier 14 is derived by connection of the latter across the defined shunt leg. Hence any reduction of resistance in the series leg, constituted by the parallel-connected resistor 22 and light-dependent resistor 23, will correspondingly increase the proportion of signal voltage supplied to the pro-amplifier, and thus pro tanto, enlarge the gain of the channel.

For control simultaneously with the described expander in channel A, a suppressor circuit is included in channel B, for example conveniently between the pre-amplifier 16 and the power amplifier 17. This device again comprises a voltage divider, wherein the series leg consists of a fixed resistance 27 and the shunt leg consists of a fixed resistance 28 connected in parallel with a series assembly of a light-dependent resistor 29 and an adjustable resistance 39. As shown, the entire voltage divider, consisting of the defined legs in series is connected across the output of the pro-amplifier 16, while the input of the power amplifier 17 is arranged to be derived from the shunt leg of the divider, including the light-dependent resistor 2?. Thus any decrease in the resistance of the shunt leg reduces the proportion of signal voltage which is applied from the divider to the power amplifier, i.e. which is transmitted in channel B, so that there is corresponding suppression or reduction in level of transmitted signals.

For control of the expander 20 and the suppressor 26, the input of a lamp-drive amplifier 32 is connected to the line of channel A, e.g. at a locality 33 which may conveniently be the input of the power amplifier 15. The output of the amplifier 32, which as stated above is a simple high gain voltage amplifier of conventional sort, having no critical design requirements, is connected to a pair of small lamps 35, 36 arranged (as with suitable covers, not shown, to screen out ambient light) to illuminate the respective light-dependent resistors 23 and 29. Indeed, if desired, in the circuit shown a single lamp can be arranged to throw light on both of the resistors simultaneously. As will be understood, the amplifier 32 is adjusted so that in response to a voice signal in channel A, an amplified output is derived sufficient to light the lamps 35 and 36 fully. Hence as soon as the desired audio signal, representing speech, appears in channel A, the amplifier 32 causes the lamps 35 and 36 to light, and in turn afiects the resistors 23 and 29, as by reducing their resistance by a factor of the order of 100, egg. from a value of the nature of 100,000 ohms to a value such as 1000 ohms. With suitable components in the voltage divider networks and 36 the result is an expansion or increase in gain in channel A, and at the same time a suppression or decrease in gain in channel B. identical expansion and cross-suppression circuits are provided for response to speech signals in channel B, e.g. as received with the microphone 12. Thus an expander 40 is connected between the microphone 12 and the preamplifier 16, being constituted as a voltage divider having a shunt leg resistor 41 of fixed value and having a series leg consisting of fixed resistor 42 and light-dependent resistor 43 in parallel. The microphone output is delivered across the entire divider, while the pre-amplifier input is derived from the shunt leg 4-1.

A cross-suppressor 46, identical with the cross-suppressor 26, is also provided, to be controlled simultaneously with the expander 40, the cross-suppressor 45 being connected in the line of channel A between pie-amplifier 14 and power amplifier 15. The suppressor 4-6 is a voltage divider identical with the first-described suppressor having a fixed resistance 47 as the series leg and having a shunt leg which consists of a fixed resistor 48 in parallel with the light-dependent resistor 49 and a variable resistor 5G in series. The pro-amplifier output is delivered across the entire voltage divider, i.e. both legs in the series, while the input to the power amplifier 15 is derived from the shunt leg that includes the light-dependent resistor.

Control of the expander 40 and cross-suppressor 46 are eilectuated from a lamp-drive amplifier 52 connected to receive a voice signal from channel B, e.g. at 53, the input of the power amplifier 17. A pair of lamps S5, 56, connected to be energized by the amplifier 52, are respectively associated with the resistors 43 and 49, for illumination of the latter, exactly as the lamps 35 and 36 previously described; alternatively a single lamp may be employed if desired. It may be noted that these lamps can be simple neon bulbs, or alternatively, incandescent lights of appropriately low power.

Hence upon occurrence of an audio signal constituting speech, e.g. of significant level, in channel B, the amplifier 52 energizes the lamps 55 and 56, bringing them to a full state of illumination, and correspondingly greatly reduces the resistance of elements 43 and 49. As a result the resistance of the series leg in the expanderdivider 40 is reduced, aifording increase of level in channel B, while at the same time reduction of resistance in the shunt leg of the suppressor-divider 46 lowers the gain, so to speak, in channel A and eliectuates the desired cross-suppression.

It will therefore be seen that the illustrated circuit provides for expansion and cross-suppression with respect to speech signals in either direction: in channel A, speech received by the microphone 10 results in a triggering signal into the drive amplifier 32 and operates the expander 20, raising the output of speaker 13 to full volume, While simultaneously operating suppressor 26, i.e. to effect crosssuppression in channel B and reduce its level somewhat below the stand-by value, as described hereinafter. Similarly a voice signal in the microphone l2 creates a triggering signal to the lamp drive amplifier 52 and operates expander 40 for channel B and cross-suppressor 46 for channel A, with identical effects in the reverse direction to the operation triggered in channel A. As will be noted, the arrangement is such that when speech signals are transmitted in one channel, the other channel is reduced below the level for operation of its expansion and cross-suppression functions, although the level is preferably such that speech is transmitted with sufiicient audibility that persons at the dominating station can clearly hear interjected responses or remarks from the other station.

It will be understood that the selection of suitable values for various elements of the expanders and suppressors may be readily effected to suit any particular circumstances and conditions including the nature of various amplifiers used in the communication lines, the line impedances, and so forth.

As an example of one system embodying the invention, the following values are appropriate for direct inclusion of the gain switcher in high impedance circuits such as may be used in a four-wire voice intercom over short or moderate distances: resistors 21 and 41 of the expanders, designated R are each 27,000 ohms and resistors 22 and 42, designated R each 100,000 ohms. In the suppressors, resistors 28 and 48, designated R are each 68,000 ohms and resistors 27 and 47, designated R each 47,000 ohms. The adjustable resistors or potentiometers 30 and 50, designate R in the suppressors, have a total resistance of 25,000 ohms, adjustment being conveniently made by shorting a greater or less amount of each resistance. As shown, these variable resistors may in some instances be conveniently ganged for adjustment together, as indicated by the dot-and-dash line 58.

The light-dependent resistors 23, 29, 43 and 49, designated R are cadmium sulfide cells as known and commercially obtainable, understood to have resistance values of about 100,000 ohms in completely dark condition. The lamps 35, 36, 55 and 56 may be suitable small discharge lamps of the so-called neon or like type, the voltage of the output of the lamp-drive amplifiers 32 being selected to assure full illumination of the lamps under the desired triggering signal. A convenient embodiment of the lamps and resistors R is in unitary assemblies, each consisting of a cadmium sulfide cell and an associated lamp with connecting housing such that the cell is exposed only to the lamp; a suitable assembly of this type is Fairchild part No. 13-25070 (Fairchild Recording Equipment Corporation). As explained above, however, it is entirely feasible to use a single lamp for each pair of light-dependent resistors, combined arrangements for control of two circuits by single lamp excitation being also commercially available. A further alternative, although ordinarily less convenient, is to have not only separate lamps but separate lamp drive amplifiers.

In the circuit example described above, the microphones 10 and 12 are high impedance devices, and in any case are preferably of the so-called long-throw type. Such devices, having high sensitivity over a limited but adequate angle, may be placed in relation to the loudspeaker so as to have a cooperative effect toward avoidance of acoustic feedback. The use of such microphones, however, is not alone adaquate to avoid howl in the system, i.e. in the absence of the present features of expansion and cross-suppression.

In initiating operation of the system of FIG. 1, the gain of each channel is first set for equal levels, i.e. with no expansion or suppression taking place in either channel. These levels are selected to be sufficiently below the feedback threshold to provide a margin of safety.

It will be understood that without the gain switching herein disclosed, such settings provide insufficient levels for sustained listening unless microphones are placed very close to the persons talking. In continuing the initial adjustment, the cross-suppressors are then brought into play at maximum level in such manner as to squelch the channel that is not speaking. Under such circumstances, the cross-suppressors function in response to speech in the opposite channel, but normal, satisfactory communication is impossible both because of absence of expansion to a high level and particularly because the complete suppression prevents an interjected remark or response from being heard so long as the speaking channel dominates the system.

The expander circuits in each channel are then switched on to provide normal listening levels, eg 14 db above the stand-by or quiescent gain of the system in the specific circuit shown. It will be understood that appropriate switching means (not shown), as in the connections tem. Greater cross-suppression can be employed, if desired, but the special advantage of the invention is that a setting may be made which will permit low volume speech in the opposite direction and at the same time will avoid feedback both then and at times when the system is quiescent. Indeed as explained above, the system is origi nally set or adjusted so that in its stand-by, open condition, the levels, including the stand-by conditions of the expanders and suppressors, are such as to avoid feedback. a

In the illustrated system, each of the signal-driven lamps changes the resistance of the accompanying cell over a range from a value of the order of 100,000 ohrns in darkness to a value of the order of 1000 ohms in full brightness, so that the division ratio of the corresponding voltage divider changes accordingly. It is preferred that each lamp-drive amplifier (32 and 52) have sufficient sensitivity to provide full brightness even when a relatively weak voice signal is sensed. Under such circumstances (requiring no variability of lamp brightness), the attack and recovery time characteristics of the cadmium sulfide cell and its associated circuitry determine the dynamic behavior of the gain switcher. Specifically, the

response to a voice signal, through illumination of the lamps, is extremely rapid (e.g. of the order oflO milliseconds), as explained above. This advantage is enhanced, in the circuit shown, by the positive feedback nature of-the control, in that the triggering signal is derived, in each line, at a locality beyond the triggered expander. On recovery, i.e. when speech ceases in a channel, the restoration of expansion and cross-suppression to stand-by levels is ordinarily slower, e.g. of the order of one or a few seconds, with advantage also as explained.

It may be noted that the illustrated circuit includes parallel and/or supplemental resistor arrangements in the divider legs that contain the light-dependent resistors, as with variable resistance (in the suppressors) to afford adjustability, and also to permit desired accomplishment of appropriate total resistance values in the dividers. In suitable circumstances, parallel resistors may not be required, or other network combinations may be employed, as will be readily understood in the light of conventional principles of resistive networks.

The so-called expansion in each circuit is of the nature of eliminating or overcoming the initial insertion loss of the variable voltage divider; the term expansion is en ployed as a convenient designation although there may advantageously be no discrimination as to degree of gain change among different control signal levels sensed by the lamp drive amplifiers. In the illustrated circuit, the insertion loss of the expanders is about 14 db, being reduced to nearly zero when the lamps are triggered. The cross-suppressors (being also variable voltage dividers) are designed to have an initial insertion loss of about 6 db, i.e. when the control lamps are dark. This loss is increased by illumination of the lamps to reduce the resistance of the shunt legs in the dividers, the degree of suppression, i.e. amount of loss, being variable by adjustrnent of resistors R which change the over-all resistance in the'shtlnt legs.

In each channel, the triggering signal is sensed at a point beyond the location of the cross-suppressor in such channel, in the direction of signal travel from microphone to speaker. This preferred arrangement is of special importance, because the microphone at the listening station fhears the loud speaker, and might well trigger expansion in the'dormant channel if the sensing point for the triggering signal in such channel were established ahead of the cross-suppression device.

Reasonable precautions, as by proper selection and placement of microphones and speakers, are helpful for increasing the acoustic isolation of these elements, thereby improving the naturamess of simultaneous two-way conversations with less cross-suppression than might otherwise be employed. On the other hand, limitation to a minimum cross-suppression may not always be desirable, inasmuch as the cross-suppression has the further function of subduing so-called back echo, i.e. that heard by the person talking.

Systems relying on cross-suppression alone, without the series expansion, are not at present deemed satisfactory, especially if desirably high gain is attempted in the two channels. Under an arrangement using only cross-suppression, the quiescent gain of the system, in stand-by condition, would then have to exceed the threshold of feedback. Likewise, the employment of expansion without cross-suppression is generally ineifective for the desired results; in such case the feedback point will be exceeded regardless of how low the quiescent gain of the system may be set. That is to say, the expanders are trigg red simultaneously in both channels when the microphone at the listening location hears the loud speaker, causing the cumulative gain of the two channels to exceed the feedback threshold, if the expanders have been set to give adequate listening levels.

As indicated above, the system is applicable to a variety of twoway, continuously open communication systems (wired or otherwise) and it will be'fuily understood that by appropriate selection of circuit components or parameters in accordance with known principles, the voltage dividers or equivalent means can be adapted for use in circuits of any desired character. For example, the system is well suited for long line conference systems or the like, as in combination with conventional 600-ohm broadcast-quality amplifier components, and in association with hybrid networks so as to permit use of a two-wire equalized telephone line. Thus FIG. 2 shows a simplified, block diagram of one such system, wherein the several microphones, loudspeakers, voltage pro-amplifiers, line amplifiers, and power amplifiers will be readily recognized without specific description, i.e. as corresponding to similar parts in FIG. 1 (except that FIG. 1 shows no line amplifiers).

In this arrangement of FIG. 2, the lines constituting channels A and B from station I are combined for a single pair, equalized line 641 by suitable hybrid coils of conventional type at 61, and the channels are re-established for station II through further hybrid coils 62, the function of transmission of signals in opposite directions through the line 60 being entirely conventional for such systems. In this particular arrangement, the expander 64 for channel A is situated in the separate part of such line at station II, ahead of the power amplifier 65, and is triggered by a signal derived in such channel, just ahead of the expander 64, as indicated by the lamp drive amplifier 66 and its control line 67. The cross-suppressor 58 for channel B, triggered from the same lamp-drive amplifier d6, through control line 69, is also situated at station II just beyond the pro-amplifier It? in channel B.

Similar locations and connections are provided at station I for the expander 71 in channel B, the suppressor '72 in channel A and the lamp-drive amplifier '73, the latter being triggered from a point in channel B ahead of the expander 71 and being arranged to control the latter and the cross-suppressor '72, respectively through lines '75 and 76.

As will be apparent, the basic function of the system in FIG. 2 is the same as in FIG. 1. Voice signals received at the microphone at station I trigger the expander 64 of channel A (in use) and likewise the cross-suppressor 68 of channel B. Upon a voice signal in the opposite direction, i.e. at the microphone of station II when chan nel A is silent, the lamp-drive amplifier 73 at station I is excited, triggering the expander 71 for channel B and the cross-suppressor 72 for channel A. This arrangernent represents a highly simple adaptation of the system to a long line arrangement. It may be noted however, that the suppressors are located, in efiiect, at the near or transmitting end of each line, so that they do not have the desirable function of suppressing undue line noise should such arise under faulty circumstances or poor conditions of the transmission line.

Alternative arrangements are readily conceivable for communication systems of the above type (using long, equalized lines), one such modification being shown in FIG. 3. Here again the microphones, speakers, preamplifiers, line amplifiers and power amplifiers for channels A and B at stations I and II will be recognized as similarly arranged to preceding views, without specific description. Likewise, the connections of the two channels with the hybrid coil system 61, 62 for accommodation to a single pair line 60 may be as explained above respecting FIG. 2. In this circuit, the expander 80 for channel A is disposed at station I (as in FIG. I), conveniently ahead of line amplifier 81, while the corresponding, principal cross-suppressor 82 for channel B is located at station II, between the line amplifier 33 and the main transmission line 60. These devices are triggered by separate lamp-drive amplifiers, viz. amplifier 84 at station I, sensing a point in channel A beyond the line amplifier 81, and an amplifier 85, sensing a corresponding point in channel A at station II just beyond the long line; the amplifier 84 controls expander 80 and amplifier 85 controls suppressor 82.

Similarly the expander 88 for channel B is located at station II, ahead of the line amplifier 83, and is controlled by lamp-drive amplifier 89 which receives its triggering si nal from channel B at a locality beyond suppressor 82, is just ahead of the main line 60. The principal cross-suppressor 90 for channel A is located intermediate the line amplifier 81 and the long line connection 61, i.e. in channel A at station I, such cross-suppressor being controlled by lamp-drive amplifier 92, deriving its signal from channel B at station I, just beyond the long line.

The arrangement of FIG. 3 also preferably includes auxiliary cross-suppressors 94 and 96, intended to suppress line noise, and avoid unpleasantness for the person talking, due to hearing interference from a faulty line, as might be occasioned in some cases as explained above. The auxiliary cross-suppressor 94 in channel B is situated at station I, ahead of the speaker amplifier 97, while the cross-suppressor for channel A is correspondingly situated in the latter channel at station II, ahead of the speaker amplifier 99. These cross-suppressors 94 and 96 can respectively be controlled or triggered by the lamp-drive amplifiers 84 and 89, thus simplifying the circuitry.

In use, the two suppressors in each channel are appropriately adjusted so that together they accommodate the total desired suppression for Voice and likewise avoid undue reproduction of line noise. The principal suppressors 32 and 90 should, of course, be adjusted to provide sufficient action as to avoid unwanted triggering of the expander in the same channel when the microphone of that channel hears speech signals from the adjacent loud speaker of the other channel. In function, speech signals in channel A trigger the expander 80 for that channel and likewise the cross-suppressors 82 and 94 for channel B. Similarly, speech signals in channel B trigger the expander 88 and the cross-suppressors 90 and 96. It will be appreciated that if line noise or similar problems are not of consequence the auxiliary cross-suppressors 94 and 96 may be omitted. I r i In conclusion, the gain switching system as disclosed above ahas been found to afford highly satisfactory results in two-way conference systems, enabling conversation between separated localities to be carried on at high levels of reproduction in a natural manner, with a desirable minimum of background noise, back echo and the like, and entirel' without difficulty due to acoustical feedback. Persons participating in such converstation, at each station, have wide freedom of location or movement about the room. There is no undue pick-up or" casual noises, as when close microphones are employed. Speech can be transmitted to each station at natural and indeed at better than normal levels, yet advantageously {as explained) without completely blocking out transmission in the reverse direction. The arrangement, moreover, is ideally suited for accompaniment to two-way television, people at both ends of the line then having the full illusion of being present together with, and talking directly to, all of the persons at the other end.

it is to be understood that the invention is not limited to the specific structures and operaions herein disclosed but may be embodied in other Ways without departure from its spirit.

I claim:

1. In a two-way audio communication system, in combination, first and second communication channels be tween two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means connected in t e first channel, in cluding a light-dependent resistor and arranged to respond to change of ilumination on said resistor for increasing the gain of said first channel suppressor means connected in the second channel, including a light-dependent resistor and arranged to respond to change of illumination on said last-mentioned resistor for reducing the gai of said second channel, and means connected to the first channel and including illuminating means, for changing illumination of said light-dependent resistors, to operate said expander means and suppressor means, in response to audio communication signals in said first channel.

2. In a two-Way audio communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, two expander means respectively connected in the channels, each expander means including gain-controlling impedance means connected in the channel of such expander means, said impedance means including a lightdependent resistor and such expander means being constructed and arranged to respond to change of illumination on said resistor for increasing the gain of the channel of such expander means, two suppressor means respectively connected in the channels, each suppressor means including gain-controlling impedance means connected in the channel of such suppressor means, said last-mentioned impedance means including a light-dependent resistor and such suppressor means being constructed and arranged to respond to change of illumination on said last-mentioned resistor for reducing the gain of the channel of such suppressor means, eans connected to the first channel at a locality between its suppressor means and its reproducing transducer, and including illuminating means, for changing illumination of the light-dependent resistors of the expander means of the first channel and the suppressor means of the second channel, to operate said last-mentioned expander and suppressor means, in response to audio communication signals in said first channel, and means connected to the second channel at a locality between its suppressor means and its reproducing transducer, and including illuminating means, for changing illumination of the light-dependent resistors of the expander means of the second channel.

and the suppressor means of the first channel, to operate said last-mentioned expander and suppressor means, in response to audio communication signals in said second channel.

3. A system as defined in claim 2, in which the illuminating means of the means connected to the first channel comprise two lamps separately associated with the expander means of the first channel and the suppressor means in the second channel, said means connected to the first channel comprising two lamp-controlling amplifiers arranged to respond to audio signals in said first channel and respectively to control the illumination of said lamps.

4. A system as defined in claim 3, which includes sup plemental suppressor means connected in the second channel at the station where said second channel terminates with a reproducing transducer, said illuminating means of the means connected to the first channel comprising a lamp which is arranged to control said supplemental suppressor and which is controlled in illumination by the lamp-controlling amplifier which governs the expander means in the first channel, for operation of said supplemental suppressor means in response to an audio signal in the first channel.

5. In a two-way voice communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means including gain-controlling impedance means connected in the first channel, said impedance means including a light-dependent resistor and said expander means being constructed and arranged for control of said impedance means by said resistor for increasing the gain of said first channel when said resistor is illuminated, suppressor means including gain-controlling impedance means connected in the second channel, said impedance means including a light-dependent resistor and said suppressor means being constructed and arranged for control of said impedance means by said last-mentioned resistor for reducing the gain of said second channel when said last-mentioned resistor is illuminated, and means connected to the first channel and including lamp means optically associated with aid light-dependent resistors, for energizing said lamp means in response to voice signals in said first channel, to illuminate said resistors for operation of said expander means and suppressor means.

6. A system as defined in claim 5, in which the lamp means comprises a pair of lamps respectively associated with the light-dependent resistors, and the means for energizing said lamp means comprises a drive amplifier having its input connected to the first channel and arranged to deliver an energizing signal to both said lamps.

7. A system as defined in claim 5, in which the lamp means comprises a single lamp associated with the lightdependent resistors to illuminate both at once, and the means for energizing said lamp means comprises a drive amplifier having its input connected to the first channel and arranged to deliver an energi ing signal to said lamp.

8. In a two-way voice communication systennin combination, first and second communication channels between two stations, said'first. channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means connected in the first channel com prising a ligh-dependent resistor and means controlled thereby for increasing the gain of said first channel when said resistor is illuminated, suppressor means connected in the second channel comprising a ligh -dependent resistor and'means controlled thereby for reducing the gain of said second channel when said last-mentioned resistor is illuminated, and means connected to the first channel and including lamp means optically associated with said light-dependent resistors, for energizing said lamp means in response to voice signals in said first channel, to illuminate said resistors for operation of said expander means and suppressor means, said system being further characterized in that the expander means comprises a voltage divider having a series leg and a shunt leg, said divider being connected in the first channel to receive signals across the two legs in series and to deliver signals across the shunt leg, said first-mentioned light-dependent resistor being connected in the series leg and being adapted to drop in resistance when illuminated, so as to increase the proportion of received signals which is delivered in the first channel by said divider.

9. In a two-way voice communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means connected in the first channel comprising a light-dependent resistor and means controlled thereby for increasing the gain of said first channel when said resistor is illuminated, suppressor means connected in the second channel comprising a light-dependent resistor and means controlled thereby for reducing the gain of said second channel when said lastanentioned resistor is illuminated, and means connected to the first channel and including lamp means optically associated with said light-dependent resistors, for energizing said lamp means in response to voice signals in said first channel, to illuminate said resistors for operation of said expander means and suppressor means, said system being further characterized in that the suppressor means comprises a voltage divider having a series leg and a shunt leg, said divider being connected to the second channel to receive signals across the two legs in series and to deliver signals across the shunt leg, said second-mentioned light-dependent resistor being connected in the shunt leg and being adapted to drop in resistance when illuminated, so as to reduce the proportion of received signals which is delivered in the second channel by said divider.

10. In a two way voice communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means connected in the first channel comprising a light-dependent resistor and means controlled thereby for increasing the gain of said first channel when said resistor is illuminated, suppressor means connected in the second channel comprising a light-dependent resistor and means controlled thereby for reducing the gain of said second channel when said last-mentioned resistor is illuminated, and means connected to the first channel and including lamp means optically associated with said light-dependent resistors, for energizing said lamp means in response to voice signals in said first channel, to illuminate said resistors for operation of said expander means and suppressor means, said system being further characterized in that the expander means comprises a voltage divider having a series leg and a shunt leg, said divider being connected in the first channel to receive signals across the two legs in series and to deliver signals across the shunt leg, said first-mentioned light-dependent resistor being connected in the series leg and being adapted to drop in resistance when illuminated, so as to increase the proportion of received signals which is delivered in the first channel by said divider; and wherein the suppressor means comprises a voltage divider having a series leg and a shunt leg, said divider being connected to the second channel to receive signals across the two legs in series and to deliver signals across the shunt leg, said second-mentioned light-dependent resistor being connected in the shunt leg and being adapted to drop in resistance when illuminated, so as to reduce the proportion of received signals which is delivered in the second channel by said divider.

11. A two-way voice communication system, which comprises, in combination: first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means connected in the first channel comprising a light-dependent resistor and means controlled thereby for increasing the gain of said first channel when said resistor is illuminated, suppressor means connected in the second channel comprising a ligh-dependent resistor and means controlled thereby for reducing the gain of said second channel when said last-mentioned resistor is illuminated, and means connected to the first channel and including lamp means optically associated with said light-dependent resistors, for energizing said lamp means in response to voice signals in said first channel, to illuminate said resistors for operation of said expander means and suppressor means; and which includes expander means connected in the second channel comprising a third light-dependent resistor and means controlled thereby for increasing the gain of said second channel when said third resistor is illuminated, suppressor means connected in the first channel comprising a fourth light-dependent resistor and means controlled thereby for reducing the gain of said first channel when said fourth resistor is illuminated, and means connected to the second channel and including lamp means optically associated with said third and fourth light-dependent resistors, for energizing said lamp means in response to voice signals in said second channel, to illuminate said third and-fourth resistors for operation of said second channel expander means and said first channel suppressor means; and each expander means comprises a voltage divider having a series leg and a shunt leg, said divider being connected in the channel of such expander means to receive signals across the two legs in series and to deliver signals across the shunt leg, the light-dependent resistor of such expander means being connected in the series leg and being adapted to drop in resistance when illuminated, so as to increase the proportion of received signals which is delivered in the channel of such expander means by said divider, and wherein each suppressor means comprises a voltage divider having a series leg and a shunt leg, said divider being connected to the channel of such suppressor means to receive signals across the two legs in series and to deliver signals across the shunt leg, the light-dependent resistor of such suppressor means being connected in the shunt leg and being adapted to drop in resistance when illuminated, so as to reduce the proportion of received signals which is delivered in the channel of such suppressor means by said divider.

12. A system as defined in claim 11, in which the expander means and the suppressor means in each channel are connected therein in succession along the direction of voice communication between the microphone and the loud speaker of said channel, and in which the means connected in each channel for energizing lamp means in response to voice signals is connected to such channel 14 at a locality between the suppressor means and the loud speaker.

13. In a two-way voice communication system, in combination, first and second communication channels between two stations, said first channel having a microphone connected thereto at a first station and a loudspeaker connected thereto at a second station, and said second channel having a microphone connected thereto at said second station and a loudspeaker connected thereto at said first station, each of said channels including an expander means and suppressor means connected in the channel for selectively respectively increasing and reducing the gain of said channel, each of said expander means and suppressor means comprising a resistive gain-com trolling network connected into the channel and including a light-dependent resistor, said gain-controlling network of each expander means being arranged so that the gain of the channel is increased upon illumination of the included resistor and the gain-controlling network of each suppressor means being arranged so that the gain of the channel is reduced upon illumination of the included resistor, means connected to the first channel and responsive to voice signals therein for illuminating the light-dependent resistors of the expander means in the first channel and the suppressor means in the second channel, and means connected to the second channel and responsive to voice signals therein for illuminating the light-dependent resistors of the expander means in the second channel and the suppressor means in the first channel.

14. A system as defined in claim 13, in which the expander means and the suppressor means in each channel are connected therein in succession along the direction of voice communication between the microphone and the loud speaker of said channel, and in which the means in each channel responsive to voice signals therein is connected to such channel at a locality between the suppressor means and the loud speaker.

15. In a two-Way voice communication system, in combination, first and second communication channels between two stations, said first channel having a microphone connected thereto at a first station and a loudspeaker connected thereto at a second station, and said second channel having a microphone conneced thereto at said second station and a loudspeaker connected thereto at said first station, each of said channels including an expander means and suppressor means connected in the channel for selectively respectively increasing and reducing the gain of said channel, each of said expander means and suppressor means comprising resistive gaincontrolling means including a light-dependent resistor, said gain-controlling means of each expander means being arranged so that the gain of the channel is increased upon illumination of the included resistor and the gain-controlling means of each suppressor means being arranged so that the gain of the channel is reduced upon illumination of the included resistor, means connected to the first channel between the suppressor means and the loud speaker thereof and responsive to voice signals therein for illumimating the light-dependent resistors of the expander means in the first channel and the suppressor means in the second channel, and means connected to the second channel between the suppressor means and the loud speaker thereof and responsive to voice signals therein for illuminating the light-dependent resistors of the expander means in the second channel and the suppressor means in the first channel, each of said channel-connected means comprising voice-signal-controlled amplifying means and lamp means energized thereby and associated with the light-dependent resistors which are to be illuminated by such channel-connected means, each of the expander means and suppressor means comprising a resistive voltage divider having a series leg and a shunt leg, each expander means having its light-dependent resistor in the series leg l of its divider and each suppressor means having its lightdependent resistor in the shunt leg of its divider.

16. In a two-way audio communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at a second station,

and said second channel having a transmitting transducer connected thereto at said second station and a repreducing transducer connected thereto at said first station, expander means connected in the first channel, including a light-dependent resistor and arranged to respond to illumination of said resistor for increasing the gain of said first channel, suppressor. means connected in the second channel, including a light-dependent resistor and arranged to respond to illumination of said last-mentioned resistor for reducing the gain of said second channel, each of said expander means and said suppressor means comprising a resistive voltage divider having a series leg and a shunt leg, said expander means having its lightdependent resistor in the series leg of its divider and said suppressor means having its light-dependent resistor in the shunt leg of its divider, and means connected to the first channel and including illuminating means, for illuminating said light-dependent resistors, to operate said expander means and suppressor means, in response to audio communication signals in said first channel.

17. In a two-way audio communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto. at a first station and a re producing transducer connected thereto at a second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, expander means for the first channel, including gaincontrolling impedance means connected in said first channel, said impedance means including a light-dependent resistor and said expander means being constructed and arranged to respond to change of illumination on said resistor for increasing the gain of said first channel, suppressor means for the second channel, including gaincontrolling impedance means connected in said second channel, said last-mentioned impedance means including a light-dependent resistor and said suppressor means being constructed and arranged to respond to change of illumination on said last-mentioned resistor for reducing the gain of said second channel, and means connected to the first channel and including illuminating means, for changing illumination of said light-dependent resistors, to operate said expander means and suppressor means, in response to audio communication signals in said first channel.

18. In a two-way audio communication system, in combination, first and second communication channels between two stations, said first channel having a transmitting transducer connected thereto at a first station and a reproducing transducer connected thereto at Ya second station, and said second channel having a transmitting transducer connected thereto at said second station and a reproducing transducer connected thereto at said first station, two suppressor means respectively connected in the channels, each suppressor means including gaincontrolling impedance means connected in the channel of such suppressor means, said impedance means including a light-dependent resistor and such suppressor means being constructed and arranged to respond to change of illumination on said resistor for reducing the gain of the channel, means connected to the first channel and including illuminating means, for changing illumination of the light-dependent resistor of the suppressor means of the second channel, to operate said suppressor means of the second channel, in response to audio communication signals in said first channel, and means connected to the second channel and including illuminating means, for changing illumination of the light-dependent resistor of the suppressor means of the first channel, to operate said suppressor means of the first channel, in response to audio communication signals in said second channel.

References Qited in the file of this patent UNITED STATES PATENTS 2,096,025 Bell Oct. 19, 1937 2,267,622 Mitchell Dec. 23, 1941 3,026,374 Hilliard et al Mar. 20, 1962 3,102,227 De Gier Aug. 27, 1963 OTHER REFERENCES Frerichs publication: Physical Review, vol. 76, No. 12, Dec. 15, 1949, pp. 1869-1875.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 ,l68 ,619 February 2, 1965 Stevens H. Harrison It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as correctedbelow.

Column 6, line 15, for "designate" read designated"; column 9 line 73, for "ahas" read has column 10 line 17, for "operaions" read operations line 32, after "channel" insert a comma; column 14, line 44, for "conneced" read connected Signed and sealed this 12th day of October 1965 (SEAL) Attest:

. ERNEST W. SWIDER Altesting Officer EDWARD J. BRENNER Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 5 ,l68,6l9 February 2., 1965 Stevens H. Harrison It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as correotedbelow.

Column 6, line 15, for "designate" read designated column 9 line 73, for "ahas" read has column 10 line 17, for "operaions" read operations line 32, after "channel" insert a comma; column 14, line 44, for "conneced" read connected (SEAL) Attest:

A ERNEST W. SWIDER EDWARD J'. BRENNER Commissioner of Patents A I testing Officer 

1. IN A TWO-WAY AUDIO COMMUNICATION SYSTEM, IN COMBINATION, FIRST AND SECOND COMMUNICATION CHANNELS BETWEEN TWO STATIONS, SAID FIRST CHANEL HAVING A TRANSMITTING TRANSDUCER CONNECTED THERETO AT A FIRST STATION AND A REPRODUCING TRANSDUCER CONNECTED THERETO AT A SECOND STATION, AND SAID SECOND CHANNEL HAVING A TRANSMITTING TRANSDUCER CONNECTED THERETO AT SAID SECOND STATION AND A REPRODUCING TRANSDUCER CONNECTED THERETO AT SAID FIRST STATION, EXPANDER MEANS CONNECTED IN THE FIRST CHANNEL, INCLUDING A LIGHT-DEPENDENT RESISTOR AND ARRANGED TO RESPOND TO CHANGE OF ILUMINATION ON SAID RESISTOR FOR INCREASING THE GAIN OF SAID FIRST CHANNEL SUPPRESSOR MEANS CONNECTED IN THE SECOND CHANNEL, INCLUDING A LIGHT-DEPENDENT RESISTOR AND ARRANGED TO RESPOND TO CHANGE OF ILLUMINATION ON SAID LAST-MENTIONED RESISTOR FOR REDUCING THE GAIN OF SAID SECOND CHANNEL, AND MEANS CONNECTED TO THE FIRST 